Older Xen versions used to expose some CPUID bits which are no longer
exposed by default. In order to keep a compatible behavior with
guests migrated from versions of Xen that don't encode the CPUID data
on the migration stream introduce a function that sets the same bits
as older Xen versions.

This is pulled out from xc_cpuid_apply_policy which already has this
logic present.

No functional change intended.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Changes since v3:
 - Rename function to xc_cpu_policy_make_compat_4_12.

Changes since v1:
 - Move comments and explicitly mention pre-4.13 Xen.
---
 tools/include/xenguest.h        |  4 +++
 tools/libs/guest/xg_cpuid_x86.c | 62 ++++++++++++++++++++++++---------
 2 files changed, 49 insertions(+), 17 deletions(-)

diff --git a/tools/include/xenguest.h b/tools/include/xenguest.h
index 2672fd043c..281454dc60 100644
--- a/tools/include/xenguest.h
+++ b/tools/include/xenguest.h
@@ -817,6 +817,10 @@ int xc_cpu_policy_get_msr(xc_interface *xch, const xc_cpu_policy_t *policy,
 bool xc_cpu_policy_is_compatible(xc_interface *xch, xc_cpu_policy_t *host,
                                  xc_cpu_policy_t *guest);
 
+/* Make a policy compatible with pre-4.13 Xen versions. */
+int xc_cpu_policy_make_compat_4_12(xc_interface *xch, xc_cpu_policy_t *policy,
+                                   bool hvm);
+
 int xc_get_cpu_levelling_caps(xc_interface *xch, uint32_t *caps);
 int xc_get_cpu_featureset(xc_interface *xch, uint32_t index,
                           uint32_t *nr_features, uint32_t *featureset);
diff --git a/tools/libs/guest/xg_cpuid_x86.c b/tools/libs/guest/xg_cpuid_x86.c
index 859c885c15..bcbf9576c4 100644
--- a/tools/libs/guest/xg_cpuid_x86.c
+++ b/tools/libs/guest/xg_cpuid_x86.c
@@ -432,6 +432,7 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t domid, bool restore,
     unsigned int i, nr_leaves, nr_msrs;
     xen_cpuid_leaf_t *leaves = NULL;
     struct cpuid_policy *p = NULL;
+    struct xc_cpu_policy policy = { };
     uint32_t err_leaf = -1, err_subleaf = -1, err_msr = -1;
     uint32_t host_featureset[FEATURESET_NR_ENTRIES] = {};
     uint32_t len = ARRAY_SIZE(host_featureset);
@@ -496,23 +497,9 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t domid, bool restore,
 
     if ( restore )
     {
-        /*
-         * Account for feature which have been disabled by default since Xen 4.13,
-         * so migrated-in VM's don't risk seeing features disappearing.
-         */
-        p->basic.rdrand = test_bit(X86_FEATURE_RDRAND, host_featureset);
-        p->feat.hle = test_bit(X86_FEATURE_HLE, host_featureset);
-        p->feat.rtm = test_bit(X86_FEATURE_RTM, host_featureset);
-
-        if ( di.hvm )
-        {
-            p->feat.mpx = test_bit(X86_FEATURE_MPX, host_featureset);
-        }
-
-        /* Clamp maximum leaves to the ones supported on 4.12. */
-        p->basic.max_leaf = min(p->basic.max_leaf, 0xdu);
-        p->feat.max_subleaf = 0;
-        p->extd.max_leaf = min(p->extd.max_leaf, 0x8000001c);
+        policy.cpuid = *p;
+        xc_cpu_policy_make_compat_4_12(xch, &policy, di.hvm);
+        *p = policy.cpuid;
     }
 
     if ( featureset )
@@ -916,3 +903,44 @@ bool xc_cpu_policy_is_compatible(xc_interface *xch, xc_cpu_policy_t *host,
 
     return false;
 }
+
+int xc_cpu_policy_make_compat_4_12(xc_interface *xch, xc_cpu_policy_t *policy,
+                                   bool hvm)
+{
+    xc_cpu_policy_t *host;
+    int rc;
+
+    host = xc_cpu_policy_init();
+    if ( !host )
+    {
+        errno = ENOMEM;
+        return -1;
+    }
+
+    rc = xc_cpu_policy_get_system(xch, XEN_SYSCTL_cpu_policy_host, host);
+    if ( rc )
+    {
+        ERROR("Failed to get host policy");
+        goto out;
+    }
+
+    /*
+     * Account for features which have been disabled by default since Xen 4.13,
+     * so migrated-in VM's don't risk seeing features disappearing.
+     */
+    policy->cpuid.basic.rdrand = host->cpuid.basic.rdrand;
+    policy->cpuid.feat.hle = host->cpuid.feat.hle;
+    policy->cpuid.feat.rtm = host->cpuid.feat.rtm;
+
+    if ( hvm )
+        policy->cpuid.feat.mpx = host->cpuid.feat.mpx;
+
+    /* Clamp maximum leaves to the ones supported on pre-4.13. */
+    policy->cpuid.basic.max_leaf = min(policy->cpuid.basic.max_leaf, 0xdu);
+    policy->cpuid.feat.max_subleaf = 0;
+    policy->cpuid.extd.max_leaf = min(policy->cpuid.extd.max_leaf, 0x8000001c);
+
+ out:
+    xc_cpu_policy_destroy(host);
+    return rc;
+}
-- 
2.33.0